3 * Database-backed job queue code.
5 * This program is free software; you can redistribute it and/or modify
6 * it under the terms of the GNU General Public License as published by
7 * the Free Software Foundation; either version 2 of the License, or
8 * (at your option) any later version.
10 * This program is distributed in the hope that it will be useful,
11 * but WITHOUT ANY WARRANTY; without even the implied warranty of
12 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 * GNU General Public License for more details.
15 * You should have received a copy of the GNU General Public License along
16 * with this program; if not, write to the Free Software Foundation, Inc.,
17 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
18 * http://www.gnu.org/copyleft/gpl.html
21 * @author Aaron Schulz
25 * Class to handle job queues stored in the DB
30 class JobQueueDB
extends JobQueue
{
31 const CACHE_TTL_SHORT
= 30; // integer; seconds to cache info without re-validating
32 const CACHE_TTL_LONG
= 300; // integer; seconds to cache info that is kept up to date
33 const MAX_AGE_PRUNE
= 604800; // integer; seconds a job can live once claimed
34 const MAX_JOB_RANDOM
= 2147483647; // integer; 2^31 - 1, used for job_random
35 const MAX_OFFSET
= 255; // integer; maximum number of rows to skip
40 /** @var bool|string Name of an external DB cluster. False if not set */
41 protected $cluster = false;
44 * Additional parameters include:
45 * - cluster : The name of an external cluster registered via LBFactory.
46 * If not specified, the primary DB cluster for the wiki will be used.
47 * This can be overridden with a custom cluster so that DB handles will
48 * be retrieved via LBFactory::getExternalLB() and getConnection().
49 * @param array $params
51 protected function __construct( array $params ) {
54 parent
::__construct( $params );
56 $this->cluster
= isset( $params['cluster'] ) ?
$params['cluster'] : false;
57 // Make sure that we don't use the SQL cache, which would be harmful
58 $this->cache
= ( $wgMemc instanceof SqlBagOStuff
) ?
new EmptyBagOStuff() : $wgMemc;
61 protected function supportedOrders() {
62 return array( 'random', 'timestamp', 'fifo' );
65 protected function optimalOrder() {
70 * @see JobQueue::doIsEmpty()
73 protected function doIsEmpty() {
74 $key = $this->getCacheKey( 'empty' );
76 $isEmpty = $this->cache
->get( $key );
77 if ( $isEmpty === 'true' ) {
79 } elseif ( $isEmpty === 'false' ) {
83 $dbr = $this->getSlaveDB();
85 $found = $dbr->selectField( // unclaimed job
86 'job', '1', array( 'job_cmd' => $this->type
, 'job_token' => '' ), __METHOD__
88 } catch ( DBError
$e ) {
89 $this->throwDBException( $e );
91 $this->cache
->add( $key, $found ?
'false' : 'true', self
::CACHE_TTL_LONG
);
97 * @see JobQueue::doGetSize()
100 protected function doGetSize() {
101 $key = $this->getCacheKey( 'size' );
103 $size = $this->cache
->get( $key );
104 if ( is_int( $size ) ) {
109 $dbr = $this->getSlaveDB();
110 $size = (int)$dbr->selectField( 'job', 'COUNT(*)',
111 array( 'job_cmd' => $this->type
, 'job_token' => '' ),
114 } catch ( DBError
$e ) {
115 $this->throwDBException( $e );
117 $this->cache
->set( $key, $size, self
::CACHE_TTL_SHORT
);
123 * @see JobQueue::doGetAcquiredCount()
126 protected function doGetAcquiredCount() {
127 if ( $this->claimTTL
<= 0 ) {
128 return 0; // no acknowledgements
131 $key = $this->getCacheKey( 'acquiredcount' );
133 $count = $this->cache
->get( $key );
134 if ( is_int( $count ) ) {
138 $dbr = $this->getSlaveDB();
140 $count = (int)$dbr->selectField( 'job', 'COUNT(*)',
141 array( 'job_cmd' => $this->type
, "job_token != {$dbr->addQuotes( '' )}" ),
144 } catch ( DBError
$e ) {
145 $this->throwDBException( $e );
147 $this->cache
->set( $key, $count, self
::CACHE_TTL_SHORT
);
153 * @see JobQueue::doGetAbandonedCount()
155 * @throws MWException
157 protected function doGetAbandonedCount() {
160 if ( $this->claimTTL
<= 0 ) {
161 return 0; // no acknowledgements
164 $key = $this->getCacheKey( 'abandonedcount' );
166 $count = $wgMemc->get( $key );
167 if ( is_int( $count ) ) {
171 $dbr = $this->getSlaveDB();
173 $count = (int)$dbr->selectField( 'job', 'COUNT(*)',
175 'job_cmd' => $this->type
,
176 "job_token != {$dbr->addQuotes( '' )}",
177 "job_attempts >= " . $dbr->addQuotes( $this->maxTries
)
181 } catch ( DBError
$e ) {
182 $this->throwDBException( $e );
184 $wgMemc->set( $key, $count, self
::CACHE_TTL_SHORT
);
190 * @see JobQueue::doBatchPush()
193 * @throws DBError|Exception
196 protected function doBatchPush( array $jobs, $flags ) {
197 $dbw = $this->getMasterDB();
200 $method = __METHOD__
;
201 $dbw->onTransactionIdle(
202 function () use ( $dbw, $that, $jobs, $flags, $method ) {
203 $that->doBatchPushInternal( $dbw, $jobs, $flags, $method );
211 * This function should *not* be called outside of JobQueueDB
213 * @param IDatabase $dbw
216 * @param string $method
220 public function doBatchPushInternal( IDatabase
$dbw, array $jobs, $flags, $method ) {
221 if ( !count( $jobs ) ) {
225 $rowSet = array(); // (sha1 => job) map for jobs that are de-duplicated
226 $rowList = array(); // list of jobs for jobs that are are not de-duplicated
227 foreach ( $jobs as $job ) {
228 $row = $this->insertFields( $job );
229 if ( $job->ignoreDuplicates() ) {
230 $rowSet[$row['job_sha1']] = $row;
236 if ( $flags & self
::QOS_ATOMIC
) {
237 $dbw->begin( $method ); // wrap all the job additions in one transaction
240 // Strip out any duplicate jobs that are already in the queue...
241 if ( count( $rowSet ) ) {
242 $res = $dbw->select( 'job', 'job_sha1',
244 // No job_type condition since it's part of the job_sha1 hash
245 'job_sha1' => array_keys( $rowSet ),
246 'job_token' => '' // unclaimed
250 foreach ( $res as $row ) {
251 wfDebug( "Job with hash '{$row->job_sha1}' is a duplicate.\n" );
252 unset( $rowSet[$row->job_sha1
] ); // already enqueued
255 // Build the full list of job rows to insert
256 $rows = array_merge( $rowList, array_values( $rowSet ) );
257 // Insert the job rows in chunks to avoid slave lag...
258 foreach ( array_chunk( $rows, 50 ) as $rowBatch ) {
259 $dbw->insert( 'job', $rowBatch, $method );
261 JobQueue
::incrStats( 'job-insert', $this->type
, count( $rows ) );
263 'job-insert-duplicate',
265 count( $rowSet ) +
count( $rowList ) - count( $rows )
267 } catch ( DBError
$e ) {
268 if ( $flags & self
::QOS_ATOMIC
) {
269 $dbw->rollback( $method );
273 if ( $flags & self
::QOS_ATOMIC
) {
274 $dbw->commit( $method );
277 $this->cache
->set( $this->getCacheKey( 'empty' ), 'false', JobQueueDB
::CACHE_TTL_LONG
);
283 * @see JobQueue::doPop()
286 protected function doPop() {
287 if ( $this->cache
->get( $this->getCacheKey( 'empty' ) ) === 'true' ) {
288 return false; // queue is empty
291 $dbw = $this->getMasterDB();
293 $dbw->commit( __METHOD__
, 'flush' ); // flush existing transaction
294 $autoTrx = $dbw->getFlag( DBO_TRX
); // get current setting
295 $dbw->clearFlag( DBO_TRX
); // make each query its own transaction
296 $scopedReset = new ScopedCallback( function () use ( $dbw, $autoTrx ) {
297 $dbw->setFlag( $autoTrx ? DBO_TRX
: 0 ); // restore old setting
300 $uuid = wfRandomString( 32 ); // pop attempt
301 $job = false; // job popped off
302 do { // retry when our row is invalid or deleted as a duplicate
303 // Try to reserve a row in the DB...
304 if ( in_array( $this->order
, array( 'fifo', 'timestamp' ) ) ) {
305 $row = $this->claimOldest( $uuid );
306 } else { // random first
307 $rand = mt_rand( 0, self
::MAX_JOB_RANDOM
); // encourage concurrent UPDATEs
308 $gte = (bool)mt_rand( 0, 1 ); // find rows with rand before/after $rand
309 $row = $this->claimRandom( $uuid, $rand, $gte );
311 // Check if we found a row to reserve...
313 $this->cache
->set( $this->getCacheKey( 'empty' ), 'true', self
::CACHE_TTL_LONG
);
314 break; // nothing to do
316 JobQueue
::incrStats( 'job-pop', $this->type
);
317 // Get the job object from the row...
318 $title = Title
::makeTitleSafe( $row->job_namespace
, $row->job_title
);
320 $dbw->delete( 'job', array( 'job_id' => $row->job_id
), __METHOD__
);
321 wfDebug( "Row has invalid title '{$row->job_title}'." );
322 continue; // try again
324 $job = Job
::factory( $row->job_cmd
, $title,
325 self
::extractBlob( $row->job_params
), $row->job_id
);
326 $job->metadata
['id'] = $row->job_id
;
329 } catch ( DBError
$e ) {
330 $this->throwDBException( $e );
337 * Reserve a row with a single UPDATE without holding row locks over RTTs...
339 * @param string $uuid 32 char hex string
340 * @param $rand integer Random unsigned integer (31 bits)
341 * @param bool $gte Search for job_random >= $random (otherwise job_random <= $random)
342 * @return stdClass|bool Row|false
344 protected function claimRandom( $uuid, $rand, $gte ) {
345 $dbw = $this->getMasterDB();
346 // Check cache to see if the queue has <= OFFSET items
347 $tinyQueue = $this->cache
->get( $this->getCacheKey( 'small' ) );
349 $row = false; // the row acquired
350 $invertedDirection = false; // whether one job_random direction was already scanned
351 // This uses a replication safe method for acquiring jobs. One could use UPDATE+LIMIT
352 // instead, but that either uses ORDER BY (in which case it deadlocks in MySQL) or is
353 // not replication safe. Due to http://bugs.mysql.com/bug.php?id=6980, subqueries cannot
354 // be used here with MySQL.
356 if ( $tinyQueue ) { // queue has <= MAX_OFFSET rows
357 // For small queues, using OFFSET will overshoot and return no rows more often.
358 // Instead, this uses job_random to pick a row (possibly checking both directions).
359 $ineq = $gte ?
'>=' : '<=';
360 $dir = $gte ?
'ASC' : 'DESC';
361 $row = $dbw->selectRow( 'job', self
::selectFields(), // find a random job
363 'job_cmd' => $this->type
,
364 'job_token' => '', // unclaimed
365 "job_random {$ineq} {$dbw->addQuotes( $rand )}" ),
367 array( 'ORDER BY' => "job_random {$dir}" )
369 if ( !$row && !$invertedDirection ) {
371 $invertedDirection = true;
372 continue; // try the other direction
374 } else { // table *may* have >= MAX_OFFSET rows
375 // Bug 42614: "ORDER BY job_random" with a job_random inequality causes high CPU
376 // in MySQL if there are many rows for some reason. This uses a small OFFSET
377 // instead of job_random for reducing excess claim retries.
378 $row = $dbw->selectRow( 'job', self
::selectFields(), // find a random job
380 'job_cmd' => $this->type
,
381 'job_token' => '', // unclaimed
384 array( 'OFFSET' => mt_rand( 0, self
::MAX_OFFSET
) )
387 $tinyQueue = true; // we know the queue must have <= MAX_OFFSET rows
388 $this->cache
->set( $this->getCacheKey( 'small' ), 1, 30 );
389 continue; // use job_random
393 if ( $row ) { // claim the job
394 $dbw->update( 'job', // update by PK
396 'job_token' => $uuid,
397 'job_token_timestamp' => $dbw->timestamp(),
398 'job_attempts = job_attempts+1' ),
399 array( 'job_cmd' => $this->type
, 'job_id' => $row->job_id
, 'job_token' => '' ),
402 // This might get raced out by another runner when claiming the previously
403 // selected row. The use of job_random should minimize this problem, however.
404 if ( !$dbw->affectedRows() ) {
405 $row = false; // raced out
408 break; // nothing to do
416 * Reserve a row with a single UPDATE without holding row locks over RTTs...
418 * @param string $uuid 32 char hex string
419 * @return stdClass|bool Row|false
421 protected function claimOldest( $uuid ) {
422 $dbw = $this->getMasterDB();
424 $row = false; // the row acquired
426 if ( $dbw->getType() === 'mysql' ) {
427 // Per http://bugs.mysql.com/bug.php?id=6980, we can't use subqueries on the
428 // same table being changed in an UPDATE query in MySQL (gives Error: 1093).
429 // Oracle and Postgre have no such limitation. However, MySQL offers an
430 // alternative here by supporting ORDER BY + LIMIT for UPDATE queries.
431 $dbw->query( "UPDATE {$dbw->tableName( 'job' )} " .
433 "job_token = {$dbw->addQuotes( $uuid ) }, " .
434 "job_token_timestamp = {$dbw->addQuotes( $dbw->timestamp() )}, " .
435 "job_attempts = job_attempts+1 " .
437 "job_cmd = {$dbw->addQuotes( $this->type )} " .
438 "AND job_token = {$dbw->addQuotes( '' )} " .
439 ") ORDER BY job_id ASC LIMIT 1",
443 // Use a subquery to find the job, within an UPDATE to claim it.
444 // This uses as much of the DB wrapper functions as possible.
447 'job_token' => $uuid,
448 'job_token_timestamp' => $dbw->timestamp(),
449 'job_attempts = job_attempts+1' ),
450 array( 'job_id = (' .
451 $dbw->selectSQLText( 'job', 'job_id',
452 array( 'job_cmd' => $this->type
, 'job_token' => '' ),
454 array( 'ORDER BY' => 'job_id ASC', 'LIMIT' => 1 ) ) .
460 // Fetch any row that we just reserved...
461 if ( $dbw->affectedRows() ) {
462 $row = $dbw->selectRow( 'job', self
::selectFields(),
463 array( 'job_cmd' => $this->type
, 'job_token' => $uuid ), __METHOD__
465 if ( !$row ) { // raced out by duplicate job removal
466 wfDebug( "Row deleted as duplicate by another process." );
469 break; // nothing to do
477 * @see JobQueue::doAck()
479 * @throws MWException
482 protected function doAck( Job
$job ) {
483 if ( !isset( $job->metadata
['id'] ) ) {
484 throw new MWException( "Job of type '{$job->getType()}' has no ID." );
487 $dbw = $this->getMasterDB();
489 $dbw->commit( __METHOD__
, 'flush' ); // flush existing transaction
490 $autoTrx = $dbw->getFlag( DBO_TRX
); // get current setting
491 $dbw->clearFlag( DBO_TRX
); // make each query its own transaction
492 $scopedReset = new ScopedCallback( function () use ( $dbw, $autoTrx ) {
493 $dbw->setFlag( $autoTrx ? DBO_TRX
: 0 ); // restore old setting
496 // Delete a row with a single DELETE without holding row locks over RTTs...
498 array( 'job_cmd' => $this->type
, 'job_id' => $job->metadata
['id'] ), __METHOD__
);
499 } catch ( DBError
$e ) {
500 $this->throwDBException( $e );
507 * @see JobQueue::doDeduplicateRootJob()
509 * @throws MWException
512 protected function doDeduplicateRootJob( Job
$job ) {
513 $params = $job->getParams();
514 if ( !isset( $params['rootJobSignature'] ) ) {
515 throw new MWException( "Cannot register root job; missing 'rootJobSignature'." );
516 } elseif ( !isset( $params['rootJobTimestamp'] ) ) {
517 throw new MWException( "Cannot register root job; missing 'rootJobTimestamp'." );
519 $key = $this->getRootJobCacheKey( $params['rootJobSignature'] );
520 // Callers should call batchInsert() and then this function so that if the insert
521 // fails, the de-duplication registration will be aborted. Since the insert is
522 // deferred till "transaction idle", do the same here, so that the ordering is
523 // maintained. Having only the de-duplication registration succeed would cause
524 // jobs to become no-ops without any actual jobs that made them redundant.
525 $dbw = $this->getMasterDB();
526 $cache = $this->dupCache
;
527 $dbw->onTransactionIdle( function () use ( $cache, $params, $key, $dbw ) {
528 $timestamp = $cache->get( $key ); // current last timestamp of this job
529 if ( $timestamp && $timestamp >= $params['rootJobTimestamp'] ) {
530 return true; // a newer version of this root job was enqueued
533 // Update the timestamp of the last root job started at the location...
534 return $cache->set( $key, $params['rootJobTimestamp'], JobQueueDB
::ROOTJOB_TTL
);
541 * @see JobQueue::doDelete()
544 protected function doDelete() {
545 $dbw = $this->getMasterDB();
547 $dbw->delete( 'job', array( 'job_cmd' => $this->type
) );
548 } catch ( DBError
$e ) {
549 $this->throwDBException( $e );
556 * @see JobQueue::doWaitForBackups()
559 protected function doWaitForBackups() {
566 protected function doGetPeriodicTasks() {
568 'recycleAndDeleteStaleJobs' => array(
569 'callback' => array( $this, 'recycleAndDeleteStaleJobs' ),
570 'period' => ceil( $this->claimTTL
/ 2 )
578 protected function doFlushCaches() {
579 foreach ( array( 'empty', 'size', 'acquiredcount' ) as $type ) {
580 $this->cache
->delete( $this->getCacheKey( $type ) );
585 * @see JobQueue::getAllQueuedJobs()
588 public function getAllQueuedJobs() {
589 $dbr = $this->getSlaveDB();
591 return new MappedIterator(
592 $dbr->select( 'job', self
::selectFields(),
593 array( 'job_cmd' => $this->getType(), 'job_token' => '' ) ),
594 function ( $row ) use ( $dbr ) {
597 Title
::makeTitle( $row->job_namespace
, $row->job_title
),
598 strlen( $row->job_params
) ?
unserialize( $row->job_params
) : false
600 $job->metadata
['id'] = $row->job_id
;
604 } catch ( DBError
$e ) {
605 $this->throwDBException( $e );
609 public function getCoalesceLocationInternal() {
610 return $this->cluster
611 ?
"DBCluster:{$this->cluster}:{$this->wiki}"
612 : "LBFactory:{$this->wiki}";
615 protected function doGetSiblingQueuesWithJobs( array $types ) {
616 $dbr = $this->getSlaveDB();
617 $res = $dbr->select( 'job', 'DISTINCT job_cmd',
618 array( 'job_cmd' => $types ), __METHOD__
);
621 foreach ( $res as $row ) {
622 $types[] = $row->job_cmd
;
628 protected function doGetSiblingQueueSizes( array $types ) {
629 $dbr = $this->getSlaveDB();
630 $res = $dbr->select( 'job', array( 'job_cmd', 'COUNT(*) AS count' ),
631 array( 'job_cmd' => $types ), __METHOD__
, array( 'GROUP BY' => 'job_cmd' ) );
634 foreach ( $res as $row ) {
635 $sizes[$row->job_cmd
] = (int)$row->count
;
642 * Recycle or destroy any jobs that have been claimed for too long
644 * @return int Number of jobs recycled/deleted
646 public function recycleAndDeleteStaleJobs() {
648 $count = 0; // affected rows
649 $dbw = $this->getMasterDB();
652 if ( !$dbw->lock( "jobqueue-recycle-{$this->type}", __METHOD__
, 1 ) ) {
653 return $count; // already in progress
656 // Remove claims on jobs acquired for too long if enabled...
657 if ( $this->claimTTL
> 0 ) {
658 $claimCutoff = $dbw->timestamp( $now - $this->claimTTL
);
659 // Get the IDs of jobs that have be claimed but not finished after too long.
660 // These jobs can be recycled into the queue by expiring the claim. Selecting
661 // the IDs first means that the UPDATE can be done by primary key (less deadlocks).
662 $res = $dbw->select( 'job', 'job_id',
664 'job_cmd' => $this->type
,
665 "job_token != {$dbw->addQuotes( '' )}", // was acquired
666 "job_token_timestamp < {$dbw->addQuotes( $claimCutoff )}", // stale
667 "job_attempts < {$dbw->addQuotes( $this->maxTries )}" ), // retries left
673 }, iterator_to_array( $res )
675 if ( count( $ids ) ) {
676 // Reset job_token for these jobs so that other runners will pick them up.
677 // Set the timestamp to the current time, as it is useful to now that the job
678 // was already tried before (the timestamp becomes the "released" time).
682 'job_token_timestamp' => $dbw->timestamp( $now ) ), // time of release
687 $count +
= $dbw->affectedRows();
688 JobQueue
::incrStats( 'job-recycle', $this->type
, $dbw->affectedRows() );
689 $this->cache
->set( $this->getCacheKey( 'empty' ), 'false', self
::CACHE_TTL_LONG
);
693 // Just destroy any stale jobs...
694 $pruneCutoff = $dbw->timestamp( $now - self
::MAX_AGE_PRUNE
);
696 'job_cmd' => $this->type
,
697 "job_token != {$dbw->addQuotes( '' )}", // was acquired
698 "job_token_timestamp < {$dbw->addQuotes( $pruneCutoff )}" // stale
700 if ( $this->claimTTL
> 0 ) { // only prune jobs attempted too many times...
701 $conds[] = "job_attempts >= {$dbw->addQuotes( $this->maxTries )}";
703 // Get the IDs of jobs that are considered stale and should be removed. Selecting
704 // the IDs first means that the UPDATE can be done by primary key (less deadlocks).
705 $res = $dbw->select( 'job', 'job_id', $conds, __METHOD__
);
709 }, iterator_to_array( $res )
711 if ( count( $ids ) ) {
712 $dbw->delete( 'job', array( 'job_id' => $ids ), __METHOD__
);
713 $count +
= $dbw->affectedRows();
714 JobQueue
::incrStats( 'job-abandon', $this->type
, $dbw->affectedRows() );
717 $dbw->unlock( "jobqueue-recycle-{$this->type}", __METHOD__
);
718 } catch ( DBError
$e ) {
719 $this->throwDBException( $e );
729 protected function insertFields( Job
$job ) {
730 $dbw = $this->getMasterDB();
733 // Fields that describe the nature of the job
734 'job_cmd' => $job->getType(),
735 'job_namespace' => $job->getTitle()->getNamespace(),
736 'job_title' => $job->getTitle()->getDBkey(),
737 'job_params' => self
::makeBlob( $job->getParams() ),
738 // Additional job metadata
739 'job_id' => $dbw->nextSequenceValue( 'job_job_id_seq' ),
740 'job_timestamp' => $dbw->timestamp(),
741 'job_sha1' => wfBaseConvert(
742 sha1( serialize( $job->getDeduplicationInfo() ) ),
745 'job_random' => mt_rand( 0, self
::MAX_JOB_RANDOM
)
750 * @throws JobQueueConnectionError
753 protected function getSlaveDB() {
755 return $this->getDB( DB_SLAVE
);
756 } catch ( DBConnectionError
$e ) {
757 throw new JobQueueConnectionError( "DBConnectionError:" . $e->getMessage() );
762 * @throws JobQueueConnectionError
765 protected function getMasterDB() {
767 return $this->getDB( DB_MASTER
);
768 } catch ( DBConnectionError
$e ) {
769 throw new JobQueueConnectionError( "DBConnectionError:" . $e->getMessage() );
774 * @param $index integer (DB_SLAVE/DB_MASTER)
777 protected function getDB( $index ) {
778 $lb = ( $this->cluster
!== false )
779 ?
wfGetLBFactory()->getExternalLB( $this->cluster
, $this->wiki
)
780 : wfGetLB( $this->wiki
);
782 return $lb->getConnectionRef( $index, array(), $this->wiki
);
789 private function getCacheKey( $property ) {
790 list( $db, $prefix ) = wfSplitWikiID( $this->wiki
);
791 $cluster = is_string( $this->cluster
) ?
$this->cluster
: 'main';
793 return wfForeignMemcKey( $db, $prefix, 'jobqueue', $cluster, $this->type
, $property );
800 protected static function makeBlob( $params ) {
801 if ( $params !== false ) {
802 return serialize( $params );
812 protected static function extractBlob( $blob ) {
813 if ( (string)$blob !== '' ) {
814 return unserialize( $blob );
822 * @throws JobQueueError
824 protected function throwDBException( DBError
$e ) {
825 throw new JobQueueError( get_class( $e ) . ": " . $e->getMessage() );
829 * Return the list of job fields that should be selected.
833 public static function selectFields() {
844 'job_token_timestamp',